This research project is designed to study the precise character of synaptic input to neurons in the spinal cord and thalamus of the mammalian central nervous system. Cats and monkeys will be the principal experimental animals, and light and electron microscope methods will be the chief experimental techniques. Specific axonal projections to particular neuronal systems will be identified by causing them to degenerate (Wallerian degeneration) or by labeling the parent cells with radioactive amino acids and studying the axonal projections by autoradiographic methods. Single identified neurons, such as spinal motoneurons, will be injected with dyes or radioisotopes, so that the synaptic inputs to distal dendrites of that particular cell can be determined. The neuronal systems concerned with somatic sensation will be examined during this project. The details of neuronal interaction elucidated by these studies will be correlated with known physiological properties of these systems, in order to better understand the mechanisms underlying somatic sensation.